Plant Hormone Signalling and How a Plant Decides
Plant hormones help coordinate growth, development and responses to changing conditions. Their effects depend on concentration, the responding tissue and interactions with other signals. Auxin and cytokinin balance can influence organ formation in tissue culture. Receptor-mediated signalling can change gene expression.
What a learner can do afterwards
- Name five traditionally studied plant hormone classes and describe an effect of each
- Explain how auxin and cytokinin balance can influence tissue-culture responses, including the limits of a ratio-only prediction
- Describe how receptor-mediated hormone signalling can change transcription-factor activity and target-gene transcription
1 · Read
You have met auxin in tropisms. Plants lack nerves, but also use electrical and hydraulic signals. In a young shoot lit from one side, extra auxin on the shaded side promotes elongation, bending the shoot toward light. In a horizontal root, extra auxin below inhibits elongation, so the root bends down. Tropisms are growth movements, without locomotion.
Read the shoot sequence from left to right. Roots respond differently to extra auxin.
Five traditionally studied classes are auxins, cytokinins, gibberellins, abscisic acid (ABA), and ethylene. Gibberellic acid is a gibberellin. Cytokinins promote cell division. Gibberellins promote stem elongation and can favour germination, opposing ABA's dormancy effects. Seeds need water and suitable conditions; water alone may not break dormancy. ABA promotes stomatal closure during drought. Ethylene is a gas that promotes ripening in some fruits and can promote leaf senescence.
Hormone effects depend on concentration, tissue and other signals. In many responsive tissue cultures, more auxin relative to cytokinin favours roots; more cytokinin relative to auxin favours shoots. This tendency is not universal. Absolute doses, species and culture conditions also matter. Equal ratios at different doses need not give equal responses.
This is a tendency under suitable culture conditions, not a guarantee.
Auxin from the shoot tip travels down the stem and helps limit side-bud growth. This apical dominance involves other signals, including cytokinins and strigolactones. Auxin acts indirectly on buds; one hormone ratio does not explain the whole process. Removing the tip can release side buds to grow.
Hormones act locally or reach other tissues, where receptors detect them. Binding changes intracellular signalling proteins. For gene-expression responses, this alters transcription-factor activity and target-gene transcription in the nucleus. The hormone need not enter the nucleus. Other responses are faster: ABA can change ion-channel activity during stomatal closure.
Hormone arrival does not guarantee a response. Cells need a working receptor and downstream pathway. Two tissues can respond differently to the same hormone concentration. If only one responds, receptor or pathway differences are possible explanations. Further tests are needed to identify the cause.
Hormone responses depend on concentration, tissue and interacting signals. Receptors connect hormone detection to cellular responses, including changes in gene transcription.
Lesson, questions and flashcards written for this dataset. OpenStax plant sensory systems chapters were consulted for background facts.
Readings and flashcards written for this dataset, with OpenStax chapters consulted for background facts. Linked videos are by CrashCourse and TED-Ed, including Valentin Hammoudi's plant-defence lesson.
More to read
Why the responding tissue matters
A hormone's effect depends on where it acts. Consider a young shoot receiving light from one side. Auxin becomes more abundant on the shaded side. Cells there elongate faster, so the shoot curves toward the light. The bending is movement caused by growth; the whole plant does not travel to a new location.
Now consider a horizontal root responding to gravity. Extra auxin on its lower side inhibits elongation there. The upper side elongates faster, and the root curves downward. More auxin therefore does not mean faster elongation in every tissue.
An explanation of a tropism should connect three observations: the direction of the stimulus, the hormone's unequal distribution and the difference in growth between the two sides. Naming auxin without describing the responding tissue leaves the direction of the bend unexplained.
What a hormone ratio leaves out
In many responsive tissue cultures, treatments with more auxin relative to cytokinin favour roots. Treatments with more cytokinin relative to auxin favour shoots. Researchers can use this tendency when choosing culture conditions, but it does not predict every result.
A ratio describes relative concentrations. It does not tell you the absolute concentration of either hormone. Two treatments may share a ratio while exposing cells to different doses. Species, tissue type and other culture conditions can also affect organ formation.
An intact plant adds further interactions. Auxin supplied by the growing shoot tip helps limit side-bud growth indirectly. Cytokinins and strigolactones also participate in branching control. Removing the tip can release side buds, but this response is not fully explained by applying one tissue-culture ratio rule to the whole plant.
Detecting a hormone and responding to it
Hormone transport and hormone response are separate parts of an explanation. A hormone may act near its source or reach another tissue. A responding cell detects it through a receptor, which changes the activity of signalling proteins inside the cell.
Some pathways alter transcription factors. These proteins regulate transcription of target genes in the nucleus. Changed transcription can affect which proteins the cell produces. The hormone itself need not enter the nucleus for this route to work.
Other responses act through proteins already present. ABA signalling can alter ion-channel activity during stomatal closure. A transcription-only account would miss this route.
If two tissues receive the same hormone concentration but respond differently, their receptors or downstream pathways may differ. This is a possible explanation, not a diagnosis from that observation alone.
Hormones also participate in defence. Herbivore damage can activate jasmonate-related responses. This connects hormone signalling with the linked plant-defence lesson, whose wider subject also includes physical barriers.
Distinguishing dryness from dormancy
A dry seed cannot germinate without taking up water. Dormancy is a different restraint: a dormant seed may remain ungerminated even when conditions would otherwise permit germination. Dryness alone does not establish that a seed is dormant.
ABA helps establish and maintain dormancy. Gibberellins can favour germination and oppose ABA's dormancy-promoting effects. Water is necessary, but adding it does not guarantee that every seed will germinate. The seed's condition and other environmental conditions also matter.
Suppose you compare germination under two treatments. Counting germinated seeds tells you about the outcome. It does not directly measure hormone concentrations or show which signalling pathway caused the difference. To explain the result through a particular hormone, you need evidence that tests that mechanism.
A careful record therefore separates what was measured from the proposed explanation. Seed counts are observations; a claim about altered hormone signalling requires further support.
Explore more
2 · Watch
Take it off screen
Where it sits
Optional connections
This opens up
Nothing builds on it yet.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.